The present invention relates to an optical filter (plate) provided at the front surface of a solid image-capturing element (e.g., a CCD) or the like and an optical device mounted with this optical filter.
A CCD employed in a digital camera or the like converts a two-dimensional optical image having specific spatial frequency characteristics to an electrical signal by sampling it in time sequence. If the spatial frequency of the optical image is equal to or less than the Nyquist frequency (a frequency equal to ½ of the sampling clock), alias signal, moire and the like are not generated. For this reason, it is necessary to restrict the spatial frequency of an optical image input to the CCD to be equal to or less than the Nyquist frequency. In addition, it is necessary to cut infrared light to prevent it from affecting the characteristics of the CCD. Thus, an optical filter plate achieved by laminating a birefringent plate (an optical low-pass filter), an infrared-cutting filter plate and the like must be provided at the front surface of the CCD.
An optical filter in the prior art is formed in a simple parallelepiped shape, since the optical filter, which must undergo the cutting process, cannot be formed in a circular shape and the orientation of the birefringent plate needs to be regulated. For instance, an external shape achieved by providing two birefringent plates enclosing a 1/4 λ plate and an infrared-cutting filter plate, which is constituted of the individual plates formed in shapes identical to one another, is bound to be a rectangular parallelepiped.
Since a sufficient space for accommodating a holding member is required along the direction of the optical axis to hold such a rectangular parallelepiped optical filter plate, there is a problem in that it is difficult to provide the optical filter plate within the limited space. For instance, rigorous dimensional restrictions are imposed along the direction of the optical axis if the optical filter is to be placed between an optical lens and the solid image-capturing element or between a mechanical shutter and the solid image-capturing element, which greatly reduces the degree of freedom afforded in design. In addition, while it is conceivable to use glue to paste the optical filter plate to the protective glass of the solid image-capturing element without employing any holding member, it is difficult to implement quality control to ensure that there is no dust or air bubbles present when the optical filter is being laminated, and there is a problem in that if dust or the like is discovered after the optical filter is glued, the problem cannot be corrected. In particular, this will result in a serious increase in the production costs when a high-density pixel solid image-capturing element with a high unit price is used.